Stereophonic amplifiers and output transformers



May l5, 1962 N. H. cRowHuRsT 3,035,120

sTEREoPHoNIC AMPLIFIERS AND OUTPUT TRANsFoRMERs FIG.

JNVENTOR. /VOAMM/V Pah/#0R57 May 15, 1962 N. H. cRowHURsT 3,035,120

STEREOPHONIC AMPLIFIERS AND OUTPUT TRANSFORMERS Filed NOV. 9, 1960 2 Sheets-Sheet 2 /Ji FIG. 2.

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M+ rwwevs United States Patent O 3,035,120 STEREOPHONIC AMPLLFIERS AND OUTPUT TRANSFORMERS Norman H. Crowhurst, Bayside, N.Y., assignor to Columbia Broadcasting System, Inc., New York, N.Y., a corporation of New York Filed Nov. 9, 1960, Ser. No. 68,206 Claims. (Cl. 179-1) The present invention relates to stereophonic amplifiers and output transformers.

In the realistic reproduction of sound it has become a matter of recognized importance to create the stereophonic effect by providing two or more speakers that are spaced apart in front of the listener. Each speaker is supplied with audio signals from stereophonic recordings or from microphones judiciously placed -in relation to the sources of sounds so as to preserve the sense of direction identified originally with each of the sounds to be reproduced. Elaborate systems have been used for this purpose, including two completely separate stereo channels, each having its own amplifier and speaker. For desirable standards of fidelity, each channel has included separate speakers for the high and low frequency ranges, and each channel has included its own frequency-separation network for selective coupling to such speakers.

Duplicate stereo channels represent duplications of expensive components. It has heretofore been proposed that the relatively narrow range of low-frequency signals can be reproduced by a common low-frequency speaker. The three-dimensional sterophonic eiect is largely preserved by means of separated satellite speakers energized only by the highs from separate stereo amplifying channels. This proposal, while representing an important economy, nevertheless involves a considerable amount of eapense that is proved unnecessary by the present invention.

An object of the present invention is to provide novel coupling means in a stereo amplifier that is economical yet highly effective for accomplishing the combined purposes of separating highs and lows in the signals from separate stereo channels, of maintaining stereo separation between the highs fromthe two channels, and of providing common low-frequency output from both channels for a common low-frequency speaker. A further object of the invention resides in novel coupling means wherein all these purposes are carried out compatibly, without any serious disturbing interaction. A further object is to provide novel coupling means for the combined functions mentioned, wherein the coupling means is of minimized complexity, of correspondingly greater reliability, and of lower cost than comparable circuit networks heretofore used.

Three illustrative embodiments of the invention for accomplishing the foregoing objects are described inV detail below. Each utilizes a coupling unit consisting of a novel transformer, and a capacitor. The coupling unit separates the highs of each stereo channel from the lows and it achieves a high degree of stereo separation between the highs, and it furtherY provides output connections for supplying the combined lows of both channels to the common low-frequency speaker.

In each of these embodiments, the transformer includes three speaker-energizing windings separated from each other, but linked by a common magnetic core. The transformer in two of the embodiments includes two primary windings that are tightly coupled to respective secondary windings, and both of these windings are loosely coupled to a third winding. In the third embodiment of the` invention, no impedance transformation is needed, and the primary windings are omitted. There are three windings on a common core in all three embodiments, the third 3,035,120 Patented May 15, 1962 winding being loosely coupled to the other two. -As a convenient generic term, the core and winding assemblies in all of the embodiments are herein termed transformers.

The third winding of the transformer in each form described is shunted by a capacitor whose size is dependent upon the other circuit impedances and upon the desired crossover frequency, to separate the highs from the lows This winding energizes the common lowfrequency speaker in each of the forms to be described.

The two stereo windings, while on the same magnetic core as that of the third winding, are separated from each other suiciently to promote high-frequency isolation between the stereo channels. The use of a single transformer in each coupling unit, with its common-core linking both stereo windings to the common third winding, represents important advantages. It involves a minimum number of parts, and of assembly costs; and it assures a high degree of symmetry in the coupling between the channels and the speakers without critical selection of components such as would be entailed if separate circuit networks were used. Further, it maintains a high level of isolation between the stereo signals without critical dependence on the character of the load (whether markedly resistive or inductive) connected to the coupling unit.

Each stereo channel can utilize a pair of push-pull amplifier tubes or transistors at the output end, as will appear. However, it has been found that the novel transformer operates within reasonable limits of distortion even where each channel has only a single-sided output, that is, where each channel uses only one tube or transistor in its output stage. This is in part attributable to the fact that the direct currents supplied to the amplifying devices through the corresponding transformer windings are po.- larized so as to set up mutually opposing magnetic fields in the magnetic core loop (thus avoiding saturation), and it is in part attributable to the fact that the lowfrequency components of stereo input are oppositely phased relative to each other in the two stereo channels and behave in the novel stereo output transformer approximately in the same manner as push-pull signals conventionally supplied to the primaries of an ordinary pushpull output transformer. It is legitimate to consider the low-frequency audio signals as oppositely phased, since these lows are inevitably picked up originally in this phase relationship, and in a reasonably symmetrical pair of stereo channels the phase opposition is preserved. Moreover, it is usual for both of the stereo channels to carry a prominent component of monophonic information. This factor contributes to the success of the novel coupling unit, but is not essential to its satisfactory operation.

For minimizing distortion, particularly where singlesided output stages are used in the stereo channels, and to enhance stereo separation between the highs, negative feedback connections in the stereo amplifying channels are highly desirable. For this purpose, a reference connection is made to the midpoint of the common lowfrequency 4output winding in two of the disclosed transformers. If this were done with a simple center-tap in an ordinary winding for the common lows, other distortion components might Well develop such as frequencydoubling and high-frequency resonances. These difficulties are connected with high-frequency stereo performance, but they arise in the low frquency common coupling circuit. Those distortion effects have been avoided, pursuant to an important specific feature of the invention, by utilizing a biilar winding in the low-frequency common output circuit. The halves of such winding are extremely closely coupled and eliminate any troublesome leakage reactances at this point. Any such leakage reactances, with capacitances that exist between windings, tend to introduce objectionable resonances.

Voutput pentode 36 by coupling capacitor 32.

The parts of the bifilar winding are connected t each other so as to afford a common ground, simplifying connection of separate feed-back circuits in the two stereo channels. Negative feedback of the full-range signal of each channel may be used for minimizing distortion. However, the available frequency selective output points can be used in negative feedback loops for bass and treble gain control which thus further improves fidelity. The tight-coupled halves of the 'biiilar Winding with their common connection as a reference point makes this effective without introducing spurious effects in the main functions of the transformer, and it simplifies the connections of the feedback loops to the transformer.

Accordingly, an additional object of the invention resides in the construction of transformers that are especially suited to use in stereo systems of the class described.

The nature of the invention will be more fully appreciated and further objects and novel features will "be apparent from the following detailed description of various embodiments shown in the accompanying drawings. In those drawings:

FIG. l is a wiring diagram of a presently preferred embodiment of the invention;

FIG. 2 is a somewhat enlarged cross-section of the transformer in FIG. l, together with certain of the crcuidt connections of FIG. l, shown diagrammatically; an

FIGS. 3 and 4 are wiring ldiagrams of modifications.

Two stereorchannels are shown in FIG, l, a right channel and a left channel 12, having stereo input at points 14 and 16. A full range of audio-frequency signalsrare supplied at these input terminals by stereo pre-amplifiers (if needed) and these are ordinarily energized by stereo pickups from a recording. Stereo signal input might alternatively come from microphones suitably arranged relative to the sound sources.

inasmuch as channels 19 and 12 are identical, only the components of channel 10 are specifically described, the corresponding components in channel 12 being identified 'by corresponding primed numbers. Signal from input terminal 14 is supplied to triode amplifier 18 by gain-control potentiometer 20 via coupling capacitor 22. Amplifier 18 has a grid-return resistor 24, a plate load resistor 26, and a self-bias resistor 28. Triodes 13 and 18 are sections of a dual triode.

Signal from the anode of amplifier 18 is coupled to Output pentodes 3l) and 30' have a common self-bias resistor 34 and bypass capacitor 36. Each of these pentodes serves as a single-sided output amplifier in its channel. A common output transformer 3S provides output coupling from both pentodes and 30 to two high-frequency speakers 31 and 31 and to a common low-frequency speaker 33.

A primary winding serves as the anode load of pentode 30, and primary winding 42B similarly is the load of pentode 3G. Secondary windings 42 and 42 are tightly coupled to primary windings 4l) and 4G', respec- V tively. An additional secondary winding including parts Y44 and 44' is spaced from both primary and secondary windings 4%-42 and 4t)-42'.

Consequently, there is only loose coupling from the primary and secondary windings to the additional secondary 44, 44'. Parts 44 andV 44 are the halves of a bilar winding, the start of one half being connected to the finish of the other half.

All of the windings 4i), 46', 42, 42', 44 and 44' are disposed on a common core 46, that provides a closed magnetic loop through all of these windings (FIG. 2). Windings 4i? and 42 are relatively remote from windings 49 and 42', and are thus loosely coupled to each other Y Yso as to have a'minimurn of high-frequency coupling.

Winding 40 carries the D.C. plate current of output pentode 3f) from D.C. terminal, 47, and winding 40 similarly carries the D.C. plate current of amplifier 39' from terminal 47. These windings are disposed oppo- Vsite each other on core-loop 46, the connections being quency speaker 33.

polarized so that there is normally no D.C. magnetization in the core.

A capacitor 43 is connected across the opposite terminals of bifilar winding i4-44'. Windings 42 and 44 have an equal number of turns and are connected by a lead Sil so that these windings are in series-opposition. Consequently output terminals 52 and S4 (which are also the remaining terminals of windings 42 and 44) provide no low-frequency output.

The high-frequency components of the signal in primary winding 40, which is tightly coupled to secondary winding 42, are obviously available at terminals 52 and 56. Because of the leakage reactance resulting from the loose couplings between windings 40 and 44, there is very little of the high-frequency signal coupled-to the winding 44; and by virtue of the tight coupling between halves 44 and 44' of the bifilar winding and because of the capacitor 48 which shunts these series-connected bifilar winding portions, winding 44 does not interfere with the eicient transmission of the highfrequency signals of channel 19 from terminal 56 to terminal 54. Consequently high-frequency signals from channel 1) reach terminals 52 and 54, to energize left high-frequency speaker 31. By like token, high-frequency signals from channel 10' reach terminals 52' and 54' to energize right high-frequency speaker 31.

Low-frequency signals in channels 10 and 12 are opposite in phase, with proper stereo input connections to the two channels. With the relative sense of windings 40t and 40' described above (for D.-C. magnetic field neutralization) the low-frequency signals thus simulate pushpull signals in windings 4G and 49'. Low-frequency sighals are eciently coupled Via billar winding 44-44 to terminals 54 and 54', for energizing common lowfrequency speaker 33.

Channels il) and 12, and output coupling transformer 33 are thus seen to be effective for providing two highfrequency stereo signals separately at terminals 52-54 and at terminals 5'V-54, respectively, for a pair of satellite high-frequency speakers and, independently, a common, combined low-frequency signal is available from channels 10 and 1'2 at terminals 54 and S4' for a common low-frequency speaker 33, The cross-over frequency between the high and low-frequency signals may, for example, be 250 cycles per second. Above this crossover frequency, the transformer described, together with Iby-pass capacitor 43, provides excellent stereo-signal separation so that the stereo signal in channel 10 appears at terminals 52 and 54 with only a minimal signal from channel 1li appearing at terminals 52' and 54'. Similarly, the high-frequency signal in stereo channel 12 is coupled to output terminals 52 and 54', with only a minimal portion of this signal appearing at terminals 52 and 54. Below the cross-over frequency, the low-frequency components of the stereo signals in channels 19 and I?, are combined, andthe combined signal is available at terminals 54 and 54' for the common low-fre- The circuit arrangement and output coupling means described involve leakage reactances between primary windings 4()i and 40' and secondary winding 44-44'. By virtue of any other reactances andthe inherent capacitances present in the transformer, diiliculties might develop in the form of objectionable resonances and generation of second-harmonic frequenciesin the high-frequency range. Indeed, such difficulties *might well develop with lthe circuit illustrated in the absence of very close coupling between winding portions 44 and 44. The biiilar winding arrangement of winding 44-44' is an effective way to avoid such distortion while, at the same time, providing a common reference point 56- that is important to the feedback connections detailed below.

Terminal 56which is the junction between portions 44 and 44' of the biflar secondary winding, provides a reference point for separate negative feedback signals forthe two stereo channels. Negative feed-back contributes materially to the present amplifier. Each primary Winding 40 and `40 might be energized by a pair of output amplifier tubes arranged in push-pull (see FIG. 4); but

this represents an added cost. The single-sided output stages 30 and 30 have been found quite satisfactory from the point of view of limited distortion, particularly at moderate power-output. This iidelity is enhanced through negative feed-back loops. The negative feed-back also enhances stereo separation. Negative feed-back signal is coupled to the cathode of amplifier 18 by resistor 58 and capacitor 60. Frequency-selective negative feedback is also furnished, this being adjustable for lbass and treble control. Capacitor 62 and resistor 64 provide an output point at which an adjustable treble control 66 is connected. Resistor 68 limits the maximum feed-back of high-frequency signal to the negative feed-back point at the cathode of amplifier 1S. Negative feed-back of low-frequency signals is adjustably controlled by variable resistor 70 which is connected between terminal 54 and the cathode of amplifier 18, series resistor 72 limiting the maximum amount of negative feed-back of lowfrequency signals. Resistors 28 and 76 and capacitor 78 constitute the load to which the negative feed-back signals are applied, resistor 76 being much smaller than resistor 2S in a practical embodiment. Treble-control variable resistors 66 and 66' are mechanically coupled to each other for conjoint operation, as are bass-control variable resistors 70 and 70.

It would lbe possible to omit parts 60, 62, 64, 66, 68, 70, 72, 76 and 78 and their primed counterparts and nevertheless retain full-range negative feedback through resistors 58 and 58 to resistors 28 and 28. However, the preferred embodiment of the invention demonstrates not only the unique performance of the output transformer in the circuit illustrated, -as an output coupling device, but this embodiment also demonstrates the additional advantage of improved bass control that is realized through adjustable negative feedback from the low-frequency output portions of the novel transformer. Negative feedback is separately eected of the treble components of the stereo signals, as described, for improved fidelity and stereo separation, and for treble gain control.

The actual structure of transformer 38 is represented in FIG. 2, together with the input, output and `feed-back connections of the circuit in FIG. l. In FIG. 2, the magnetic core 46 includes stacked U-shaped core laminations 46a -and I-shaped core laminations 4611. These laminations are advantageously of non-oriented silicon steel. The legs of U-shaped lamination 46a are seen to be unequal. 'Ihe long-leg sides 46c alternate with short legs 46d of other stacked U-shaped laminations. By like token, it is seen that I-shaped lamination 45!) projects across the end of the shorter leg 46d; and this projection extends in opposite directions from each lamination to the next in the stack of laminations, to abut the successive short legs 46d of the U-shaped laminations.

Winding 40 of FIG. 1 is seen in FIG. 2 to be divided into two portions 40a and 40h, respectively outside and inside secondary winding 42, to enhance the desired tight coupling. Correspondingly, winding 49' is divided into two portions, 40a and 4Gb', outside and inside secondary winding 42' on the other leg of the U-shaped portion of the core. As illustrated, the starts of windings 46h and 40b' extend to the anodes of amplifier tubes 30 and 30'. The opposite ends of the primary windings extend to B-plus as illustrated. One end of each of the secondary windings 42 extends to ground, and the other ends of these secondaries extend to the output point 52 and 52', so phased as to provide negative feed-back -for its own amplifier channel.

The illustrated division of primary windings 40 and 40 each into two parts is one way of obtaining tight coupling Ibetween primaries and secondaries. However, other winding arrangements will -be found effective. One practical arrangement is to dispose each entire primary windgizing direct-current o-f those devices.

ing, undivided, under its corresponding secondary. Other suitable winding arrangements may be substituted.

Bilar winding 44, 44 is disposed on the I-shaped core laminations 46b, the start and finish of alternate bifilar Winding portions forming the grounded terminal 56. From the phasing determined -by the connections to windings `42 and 42' (in providing negative feedback) the remaining connections of the biiilar winding are brought out as second connections for the left and the right high-frequency speakers 31 and 31', phased so that the output from the two windings 42 and 44 will cancel below 250 cycles. (This -is determined more readily without capacitor 48 connected.)

It is seen that transformer 38 and capacitor 48 provide the entire output coupling means from the stereo channels 10 and 12 to the three speakers including satellite speakers 31 and 31 and common bass speaker 33. Stereo separation between the highs is preserved, and frequency division between the stereo-separated highs and the common lows are all achieved lby the single transformer 38 and capacitor 48. Symmetry between the two channels is realized without exacting care in matching separate inductors or other components found in coupling networks used previously. A common reference point is made available for negative feed-back circuits, and frequencyselective feed-back points are likewise available for bass and treble gain control. Single-sided output stages in the stereo channels are accommodated. The stereo separation is achieved without critical dependence on the character of the load impedance, whether inductive or resistive.

A modification of the circuit in FIGS. l and 2 is illustrated in FIG. 3, in which 10U-series numbers are employed to designate parts corresponding to those in FIGS. l and 2. In FIG. 3, the stereo channels and 112 employ transistors7 or Speakers having high impedance matching other yamplifiers are used, so that the output circuit can be coupled directly to the speakers, without impedance transformation. IFor this reason, in FIG. 3 the primary windings of the transformer 38 in FIGS. l and 2. do not appear in FIG. 3. Windings 142 and 142', disposed on the opposite legs of U-shaped :magnetic core laminations, are connected directly to the amplifying devices in channels 110 and 112, so as to carry the ener- Biilar windings 144-144 is similarly mounted on the end portion of the core, shunted -by capacitor 148. The various windings are connected to each other in FIG. 3 and phased in relation to each other as in FIGS. 1 and 2, the 'signiiicant `difference being that the close-coupled primary windings of FIGS. 1 and 2 are omitted. Like feed-back connections are provided, and like lspeaker connections are included. D.-C Iblocking and high-frequency coupling capacitors 157 and 157 are interposed between transformer windings 142 and 142 and the respective high-frequency speakers 131 and 131.

Windings 142 and 144 are loosely coupled. The resulting leakage inductance limits high-frequency signal coupling from winding 142 to 144. These windings have an equal number of turns and are connected in series-opposition.

Capacitor 148 complements the transformer in establishing the cross-over frequency. Little if `any low-frequency output appears at speakers 131 and 131'. Direct current supply 159 may, for example, be interposed between the common connection of winding portions 144 and 144 and ground, for providing direct current for output transistors or other amplifiers in channels 110 and 112. Suitable feed-back connections from terminals 152, 152', 154 and 154 of the transformer `are to extend to the respective amplifier channels 110 and 112, and may include frequency-selective components as in the circuit of FIG. 1.

There is distinct economy to be realized without introducing notable distortion through use of single-sided ampliers at the output ends of channels l@ and 12. This is particularly true when it is considered that there is a substantial monophonic push-pull component in usual stereophonic signals. However, it is entirely feasible for push-pull ampliers to `be used in the output. Such an arrangement is illustrated in FIG. 4. In this embodiment, the primary windings 240 and 246 are center-tapped and the center-taps are connected to B-plus. Dual-section output amplifiers 234) and 23o' are connected to the opposite ends of primary windings 240 and Zit. A simple winding 244 is -used to provide common low-frequency signals for low-frequency speaker 233. Just as in FIGS. 1 and 2, windings 242 and 244 are connected in seriesopposition to high-frequency speaker 231 for one of the channels, and windings 242 land 244 are connected in series-opposition for energizing the speaker 231 for the high-frequencies ofthe other channel. The phasing of the Signals in the two channels is to be such in relation to windings 240 and 240 so that these primary windings complement each other in producing low-frequency signal output in Winding 244.

Negative feed-back eonnectionsare not shown in FIG. 4, but such connections can be used if desired. Greater circuit complexity is entailed and hence the bilar winding arrangement in FIGS. l and 2 and in FIG. 3 are preferred. Nevertheless, the circuit of FIG. 4 represents an embodiment of certain Ibroader aspects lof the invention.

The embodiments of FGS. 3 and 4 achieve certain of the functions and economies of the embodhnent in Fl-GS. 1 and 2, and all three, in common, include a single transformer having a core loop that couples a pair of stereo input windings loosely to ya third winding that is shunted by a capacitor. In each case, the frequency separation of yboth stereo signals into highs `and lows is accompanied by maintenance of stereo separation between the highs and by combination of the lows in a common output circuit. All three coupling arrangements are characterized 'by inherent symmetry and Iby minimal circuit complexity.

In all three embodiments the cross-over frequency is determined by the capacitor 48, 148 or 248 in conjunction with the leakage inductance between windings 42, 42 and 44, 44 relative to the connected circuit impedances. ln a practical exampleof :the dorm in FIGS. 1 and 2, transformer 33 had primaries of 3200 turns, secondaries of 110 turns each, a bilar Winding having 110 turns for each half, and the laminated core of non-oriented Isilicon steel had a cross-section of inch square. The impedance of the biiilar winding between terminals 54 and 54' was 8 ohms, and capacitor 48 was a 100 mfd. electrolytic unit. ln each of the circuits illustrated, there is a common low-frequency speaker arid a pair of high-frequency stereo speakers. These circuits can be used to advantage even with full-range stereo speakers in place of the highand llow-frequency speakers shown in the drawings.

Full-range stereo signals maybe obtained for such rhillrange stereo speakers l'from the circuits illustrated if this should be necessary, lFor example, a full-range speaker for channel 10 in FIG. 1 may be connected between terminals 52 and 54 Vand the full-range speaker for channel 12 may =be connected between terminals 52 and 5ft.

For such yfull-range speaker operation, this invention offers a special advantage that will be clear from the following. It is characteristic of dynamic speakers that they have a considerable rise 4in impedance in the region of their low-frequency resonance. In normal amplier design this results in mis-match and loss of available power at these frequencies. In the circuit of FlG. 1, by connecting a full-range speaker for channel 1i) between terminals 52 and 54T and a full-range speaker for channel 12 between terminals 52'V and 54, the impedance match changes at the crossover frequency. Above this point, the transformer matching suits the higher frequency voice coil impedance. Below it, the impedance match will rise to a higher value. Using equal turns in windings 42 and 42 Can with those in each part of the biiilar section 44 and 4d', the impedance rise will be a factor of 4 times. By using turns combinations other thanequal, Iwhich will still be effective in preserving separation above the crossover frequency just as well, any impedance rise 'factor desired may be achieved, less or more than 4 times.

It is apparent that additional variations and varied applications of the novel features -found in the embodiments described above will occur to those skilled in the art, and therefore it is appropriate that this invention should ybe broadly construed in accordance with its full spirit and scope.

What is claimed is:

1. A stereophonic amplier having right and left ampliiier channels, right and left high-frequency speakers and a common low-frequency speaker, and output coupling means between said channels and said speakers for dividing the youtput signals of said channels into high and low-frequency signals above `and below a cross-over frequency, and for maintaining stereo separation between the high-frequency signals while providing combined output of low-frequency signals from both channels, said ontput coupling means including a transformer having a magnetic core, a common winding disposed on said core and connected to said low-frequency speaker, additional windings `on said core connected at the output of said stereo channels respectively, said additional windings being relatively remote from each other and spaced symmetrically about said common winding, each of said additional windings being connected in series-opposition to said commonwinding in respective output circuits and each of said output circuits being connected to a respective one of said high-frequency speakers, and a capacitor connected across said common winding and relatively proportioned to determine the cross-over frequency.

2.. A stereophonic ampliiier having right and left amplifier channels, right and left high-frequency speakers and a common low-frequency speaker, and output coupling means kbetween said channels and said speakers for dividing the output signals of said channels into high and low-frequency signals above and below a cross-over frequency, and for maintaining stereo separation between the high-frequency signals while providing combined output of low-frequency signals from both channels, said output coupling means including a transformer 'having a Imagnetic core, a common low-frequency winding disposed on said core and connected to said lowfrequency speaker, primary vwindings on said core at the output of said stereo channels respectively, respective secondary windings on said core tightly coupled to said primary windings, each of said secondary windings `being connected in series-opposition to said common winding in respective output circuits and each of said output circuits being connected to a respective one of said high-frequency speakers, and a capacitor shunting said common winding and relatively proportioned to determine .the cross-over frequency, said primary windings being disposed relatively remote from each other and symmetrically about said common winding.

3. A stereophonic amlpliiier having right and Vlcft" amplifier channels, corresponding right and left highfrequency speakers and a common low-frequency speaker, and output coupling means between said channels and said speakers for dividing the output signals of said channels into high and low-frequency signals above and below a cross-over frequency, and for maintaining stereo separation between the high-frequency signals while providing combined output of low-frequency signals from both channels, said output coupling means including a transformer having a magnetic core, a common bilar winding having the start of one-half thereof joined to the nish of the other haifethereof, said bilar winding'being connected to said low-frequency speaker, additional windings on said core at the output of said stereo channels respectively, said additional windings being relatively widely spaced from each other and spaced symmetrically about said biiilar winding, each of said additional windings being connected in series-opposition to said bilar winding in respective output circuits and each of said output circuits being connected to a respective one of said high-frequency speakers, a capacitor shunting said bitilar winding and relatively proportioned to determine the cross-over frequency, and negative feed-back connections from said windings to said channels and including the joint between the halves of said biilar winding.

4. A stereophonic ampliiier having right and left channels, respective right and left high-frequency speakers and a common low-frequency speaker, and frequency-selective output coupling means between sm'd stereo channels and said speakers effective to maintain stereo separation of the high-frequency signals and to combine the low-frequency signals of the two channels, said coupling means including a transformer having right and left primary windings, right and left secondary windings each coupled tightly to a respective one of said primary windings, an additional secondary winding, a magnetic core providing a common loop linking said windings, said windings being spaced apart and said primary windings being symmetrically disposed with respect to said additional winding to insure loose coupling between each primary winding and said additional winding, the winding sense of said primary windings in relation to said additional secondarywinding being such as to induce aiding voltages in said additional winding in response to oppositely phased signals in the output of said channels, said additional winding being connected to each of said secondary windings in series-opposition and each pair of series-opposed windings providing a pair of terminals for a respective high-frequency speaker, a capacitor connected across said additional lwinding and relatively proportioned to determine the cross-over frequency, and a common low-frequency speaker connected to said additional secondary winding.

5. A stereophonic amplifier having right and left channels, respective right and left high-frequency speers and a common low-frequency speaker, and frequency-selective output coupling means between said stereo channels and said speakers effective to maintain stereo separation of the high-frequency signals and to combine the low-frequency signals of the two channels, said coupling means including a transformer having right and left primary windings, right and left secondary windings each coupled tightly to a respective one of said primary windings, an additional secondary winding, a rectangular magnetic core providing a common loop linking said windings, said primary windings being disposed Von opposite sides of said rectangular core and said additional winding being disposed on an end of said rectangular core symmetrically between said primary windings to insure loose coupling between each primary winding and said additional winding, the winding sense of said primary windings in relation to said additional secondary winding being such as to induce aiding voltages in said additional winding in response to oppositely phased signals in the output of said channels, said additional winding being connected to each of said secondary windings in series-opposition and each connected pair of said seriesopposed windings providing a pair of terminals for a respective high-frequency speaker, a capacitor connected across said additional winding and relatively proportioned to determine the cross-over frequency, and a common lowfrequency speaker connected to said additional secondary winding.

6. A stereophonic amplifier having right and left channels, respective right and left high-frequency speakers and a common low-frequency speaker, and frequency-selective output coupling means between said stereo channels and said speakers effective to maintain stereo separation of the high-frequency signals and to combine the low-equency signals of the two channels,

said coupling means including a transformer having a rectangular magnetic core, right and left windings on opposite legs of said core, a bilar winding on one end of said rectangular core symmetrically spaced from said right and left windings, the halves of said biiilarV winding having a joint where the start of one half is joined to the iinish of the other half, said righ winding being connected in series-opposition to one half of said bifilar winding at said joint and the opposite terminal of said one half of said bilar winding and the opposite ternnnal of said right winding being connected to said night high-frequency speaker, the other half of said biilar winding being likewise connected in series-opposition to said left winding and to said left speaker, a capacitor connected across said bilar winding for determining the cross-over frequency, and a common lowfrequency speaker connected to said bitilar winding.

7. A stereophonic amplifier in accordance with claim 6, wherein symmetrical negative feed-back loops are included lbetween said windings `and said channels, and wherein said joint between said biilar winding halves is connected to said channels as a signal reference point.

8. Frequency-selective output coupling means for coupling two stereo channels to a pair of high-frequency speakers and a common low-frequency speaker and effective to maintain stereo separation ofthe high-frequency signals and to combine the low-frequency signals of the two channels, said coupling means including a transformer having right and left primary winding, right and left secondary windings each coupled tightly to a respective one of said primary windings, an additional secondary winding, a magnetic core providing a common loop linking said windings, said windings being spaced apart `and said primary windings being symmetrically disposed lwith respect to said additional winding to insure loose coupling between each primary winding and said additional winding, said primary windings being connected to each other :at a common direct-current energizing point and having respective terminals remote therefrom for said stereo channels, respectively, the winding sense of said primary windings -in relation to said additional secondary winding being such as to induce 'aiding voltages in said additional winding in response to oppositely phased signals -at said'remote terminals, said additional winding being connected to each of said secondary windings in series-opposition and each connected pair of said series-opposed windings providing -a pair of terminals for a respective high-frequency speaker, a capacitor connected across saidaddit-ional winding and relatively proportioned to determine the cross-over frequency,` and said additional secondary winding having terminals for connection to a common low-frequency speaker.

9. Output coupling means for a pair of stereo amplifying channels, including a transformer having -a rectangular magnetic core and plural windings thereon, said core having opposite sides and opposite ends, said windings including a primary winding and a secondary winding tightly coupled thereto on each of said opposite sides and an additional winding on one of said core ends, said primary wind-ings being connected to each other to provide a common direct-current energizing terminal and having a relative winding sense to induce -aiding voltages in said additional winding when push-pull primary signals are provided, each of said secondary windings being connected in series-opposition to said additional winding for providing respective pairs of high-frequency stereo output terminals, and a capacitor connected across said additional winding and relatively proportioned to determine the cross-over frequency, said additional winding having terminals for a common low-frequency speaker for both stereo channels.

10. Output coupling means for a pair of stereo amplifying channels, including a transformer having a rectangular magnetic core and plural windings thereon, said core having opposite sides and opposite ends and said windings including -a stereo output winding on each of said core sides and a bilar winding on one of said core ends, said bifilar 'winding having the start of one half connected at a common terminal to the tinish of the other half, the stereo output windings also being connected to said terminal and said biiilar Winding halves thus connected to said stereo output windings forming two output circuits in each of which a stereo winding and a biiilar half-winding are in series-oppositiom the number of turns in the biiilar winding halves being equal to the number of turns in each stereo output winding, and a crossover-frequency determining capacitor connected across said bitilar windlng.

l1. An output transformer for a pair of stereo amplifying channels, including a rectangular core comprising U-shaped laminations of unequal-length legs and l-shaped laminations complementary to said U-shaped laminat-ions and stacked in reversed alternation to form said rectangular core, said transformer including duplicate stereo signal [windings on said core legs, respectively, and a common winding on said I-shaped larninations symmetrically disposed relative to s-aid stereo signal windings, and said stereo -windings being individually connected to said common winding in series-opposed relation.

12. An output transformer for a pair of stereo channels, including a rectangular core comprising U-shaped laminations of unequal-length legs and I-shaped laminations complementary to said U-shaped laminations and stacked in reversed alternation to form said rectangular core, said transformer including duplicate stereo signal windings on said core legs, respectively, and a bitilar winding on said I-shaped :laminations, each half of said biiilar winding being equal in number of turns to one of said stereo windings and connected to a respective one of said stereo wind-ings in series-opposition, the start of one of the bili-lar winding halves being joined -to th finish of the other.

13. An output transformer for a pair of stereo amplifying channels, said transformer `having a rectangular core comprising opposite sides and ends, a primary winding and secondary winding tightly coupled thereto on each of said core sides, a common bitilar winding on one of saidcore ends, said primary windings being connected to each other and having a relative winding sense such that push-pull signals to said primary windings will induce aiding voltages in'said biiilar winding, the start of one half of the lbifilar winding being connected to the finish of the other half of the bilar winding and said secondary windings also being connected to each other at said connectionrbetween the -bilar winding halves, Ythe winding sense of each secondary being opposite that'of a respective one of said biiilar winding halves.

14. An output transformer for a pair of stereo channels, said transformer having a rectangular magnetic core and pluralwindings thereon, said core having opposite sides and opposite ends and said windings including a stereo output winding on each of said core sides and a biiilar winding on one of said core ends, said biiilar winding having the start of one half connected at a common terminal to the finish of the other half, the stereo output windings :also being connected to said terminal and said biilar winding halves thus connected Vto said stereo output windings forming two ouput circuits in each of which a stereo winding and a bifilar half-winding are in serieslopposition, the number of turns in the bifilar winding halves lbeing equal to the number of turns in each stereo output winding. Y

15. A stereophonicY amplifier, including right and left amplifying channels vand a common output transformen'said transformer having a closed-loop core of magnetic laminations, a number of windings on said core including a common winding, and a pair of additional windings each having an output terminal and a yter,-

minal connected to a respective terminal of said common winding, and each said additional winding being series-opposed relative to said common winding and symmetrically spaced therefrom so as to produce leakage reactance between said common winding and each of said additional windings, a condenser-connected across said common winding and relatively proportioned to establish a cross-over frequency, and a pair of stereo speakers connected to said additional windings, respectively.

16. A ystereo amplifie-r in accordance with claim l5, including separate high-frequency and low-frequency adjustable negative feedback paths from the output of said transformer to each of said stereo amplifying channels, said low-frequency negative feedback paths having their input connections at the terminals of said common winding.

17. A stereophonic amplifier including right and left amplifying channels and a common output transformer for both of said channels, said transformer ha ing a closed-loop magnetic core, a bitilar winding on said core, said biiilar winding having the start of one half thereof connected at a common terminal to the finish of the other half thereof, a pair of Aadditional windings on said core remote from each other and symmetrically disposed with respect to said biiilar winding, said -bilar winding and said additional `windings having a sub-stantial leakage reactance therebetween, each of said additional windings being connected to a respective amplifying channel and to a terminal of a respective half of said biiilar winding in `series-opposition thereto, Va capacitor connected across the biiilar winding relatively proportioned to establish a cross-over frequency between low- Vfrequencies and high-frequencies, and respective low-frequency adjustable negative-feedback signal paths between the terminals of said biiilar winding and said stereo channels, each of said stereo channels 'also having a reference connection to said common terminal of Said bitilar winding.

18. A stereophonic amplifier in accordance with claim 17, additionally including a pair of high-frequency selective adjustable negative feedback paths connected from said additional windings to said amplifying channels, respectively.

19. A stereo-phonic amplifier including right and left amplifying channels and a common output transformer, said transformer having `a closed-loop core of magnetic material, three windings on said core including a common winding and an additional pair of windings on said core symmetrically disposed relative to said corn- =mon winding and arranged to have leakage reactance between saidl common winding and each of said additional windings, a condenser connected across said common winding and relatively proportioned to establish a cross-over frequency, coupling Imeans for applying output from each of said channels to a respective one of said additional windings, and a pair of full-range dynamic speakers, each speaker being connected to a respective one of said additional windings and to said common winding, the windings connected to each speaker being series-aiding, and each speaker having a rising impedance characteristic below -said cross-over frequency and said common winding having a number of turns greatly in eX- cess of each said additional winding.

2i). A stereophonic amplifier in accor-dance with claim 19, wherein said common winding has a center-tap that is connected said amplifying channels as a signal reference pointV and wherein a pair of adjustable negativefeedback bass gain control circuits are connected between the respective terminals of said common winding and the amplifying channels, respectively.

No Vreferences cited. 

